In recent years, there has been considerable interest in adapting STM apparatus and methods for use in direct access storage devices because of their potential for ultrahigh density recording.
U.S. Pat. No. 4,575,822 discloses a data storage device in which data are written by establishing physical, electrical or magnetic perturbations in a flat storage surface on a substrate. Using STM techniques, the data are thereafter read by establishing a tunneling electron current between the surface and a movable probe. The physical perturbations are created by a physical probe, a focused laser beam, an electron beam or other forms of radiation beams and particle beams, or the perturbations can be created by deposited conductive, insulative or magnetic particles. In each such case, the storage surface is damaged or marred during creation of the physical perturbation. The only way suggested for erasing physical perturbations is by bulk annealing. Note that the written data are read by measuring tunnel electron currents or sensing stored charge rather than by measuring the effects on tunneling current of changes in electronic properties, such as conductance, work function or band gap, caused by changes in state or phase of the storage material; and there is no teaching of using the STM techniques for selectively erasing data by selectively inducing a change in state of the storage surface. "Conductance", as herein used, is defined as the derivative of current vs. voltage, and may be a function of voltage.
Research Disclosure 28130 published in the September 1987 edition of the British Journal suggests use of STM technology with laser or particle-beam induced changes to write data on an optical disk by changing the state of the magnetic coating by polymerization or electro-erosion. These phase changes could involve transitions from crystalline to amorphous states. The process is directed to write-once recording with no mention of any readout mechanism; there is no teaching of a method or means for sensing changes in electronic properties or erasing the data and enabling the disk to be reused. The writing process mentioned only refers to electrons striking the media, implying the tip is negative with respect to the media; there is no suggestion of resistive heating or of the advantage of using a tip which is positive during writing to achieve advantages hereinafter described.
Copending application U.S. Ser. No. 07/090,636, filed Aug. 28, 1987 (Docket SA9-84-047X), assigned to the assignee of the present invention, discloses a process employing a film which, when melted in selectable discrete areas by a laser and cooled at one rate, changes from a crystalline to amorphous state for writing discrete data, and when heated by the same or a different laser and cooled at a different (slower) rate, erases such data. Various film materials are disclosed which exhibit a change in optical characteristics dependent upon their selected state. There is no teaching of use of STM techniques to alter any electronic property of the film for writing data, sensing the effect of the property on the tunneling current to read the data, and altering the property to erase the data. There is no disclosure as to how the data, once written, is read before erasure.